0002022626 2025-11-03 2025-11-03 0002022626 1 2025-11-03 2025-11-03 0002022626 2 2025-11-03 2025-11-03 0002022626 3 2025-11-03 2025-11-03 0002022626 4 2025-11-03 2025-11-03 0002022626 5 2025-11-03 2025-11-03 iso4217:USD pure shares S-3 S-3ASR EX-FILING FEES 0002022626 Upstream Bio, Inc. N/A N/A S-3 Upstream Bio, Inc. false Equity Common Stock, par value $0.001 per share true 0.0001381 false Equity Preferred Stock, par value $0.001 per share true 0.0001381 false Debt Debt Securities true 0.0001381 false Equity Warrants true 0.0001381 false Equity Units true 0.0001381 0.00 0.00 0.00 0.00 0.00 (a) An indeterminate amount of the securities of each identified class is being registered as may from time to time be offered hereunder at indeterminate prices, along with an indeterminate number of securities that may be issued upon exercise, settlement, exchange or conversion of securities offered or sold hereunder. Pursuant to Rule 416 under the Securities Act of 1933, as amended (the "Securities Act"), this registration statement also covers any additional securities that may be offered or issued in connection with any stock split, stock dividend or pursuant to anti-dilution provisions of any of the securities. Separate consideration may or may not be received for securities that are issuable upon conversion, exercise or exchange of other securities. In accordance with Rules 456(b) and 457(r) under the Securities Act, the registrant is deferring payment of all registration fees and will pay the registration fees subsequently in advance or on a "pay-as-you-go" basis. (b) Including such indeterminate amount of common stock as may be issued from time to time at indeterminate prices or upon conversion of debt securities and/or preferred stock registered hereby, or upon exercise of warrants registered hereby, as the case may be. See Note 1(a). Including such indeterminate amount of preferred stock as may be issued from time to time at indeterminate prices or upon conversion of debt securities and/or preferred stock registered hereby, or upon exercise of warrants registered hereby, as the case may be. See Note 1(a). Including such indeterminate principal amount of debt securities as may be issued from time to time at indeterminate prices or upon exercise of warrants registered hereby, as the case may be. See Note 1(a). Warrants may be sold separately or together with any of the securities registered hereby and may be exercisable for shares of common stock, preferred stock, debt securities, or units registered hereby. Because the warrants will provide a right only to purchase such securities offered hereunder, no additional registration fee is required. See Note 1(a). Each unit will be issued under a unit agreement and will represent an interest in two or more securities registered pursuant to this registration statement, which may or may not be separable from one another. Because the units will provide a right only to purchase such securities offered hereunder, no additional registration fee is required.